1. Field of the Invention
This invention relates to a control arrangements for optical disc drives. More particularly this invention relates to an improved servo control which extends the operating range of a closed loop mode of operation of a focus or tracking servo to a plurality of positions on a disc.
2. Description of the Related Art
In optical disc drives wherein information is stored in a plurality of spiral or concentric information tracks, a lock of a recording or reproducing beam on an information track of interest is commonly maintained by a tracking servo, for example the servo disclosed in Ceshkovsky et al., U.S. Pat. No. 4,332,022. The tracking servo responds to minimize an error signal Vp derived from the intensity of a reflected light beam returning from the optical disk medium and is given by the equation: ##EQU1## where A is a constant;
x is beam displacement from the track center; and PA1 p is the track pitch.
Terashi, U.S. Pat. No. 5,177,725, discloses a servo apparatus for expanding the pull-in range using a velocity detector for detecting the velocity of a driven element.
Kobayashi et al, U.S. Pat. No. 4,853,918, proposes an arrangement where signals from tracking pits offset from each other about the center of a track are provided to sample-and-hold circuits, and are compared with one another to yield a sawtooth signal tracking signal having discontinuities midway between tracks.
In Burroughs, U.S. Pat. No. 4,779,251 an arrangement is disclosed wherein a circuit generates a ramp waveform that is used to introduce a controlled offset into a tracking servo. The servo error signal, which is derived from pre-formatted fine tracking features, is inverted in phase when the read beam is moving between tracks. The ramp waveform is adjusted according to memorized tracking error information from previous microjumps between tracks.
In focus control applications, the focus servo operates in the negative feedback region of the focus error "S curve". Conventionally a special sequence is required, using open loop operation, to place the servo into a negative region which is surrounded by positive feedback regions. If focus is lost for any reason, the entire acquisition sequence must be repeated. This is very time consuming.